1. Field of the Invention
The invention concerns a feed device for multibeam antennas, that is to say a device for controlling the relative phase and amplitude of the various radiating elements constituting the feed array employed to illuminate the focussing devices in such a way as to constitute focussed antennas, the focussing devices comprising at least one reflector or lens.
2. Description of the Prior Art
Focussed antennas are used in satellite communications in particular. They can produce shaped or unshaped, fixed or reconfigurable, multiple beams.
Antennas of this kind must have the following properties:
maximal edge of coverage gain and a high potential of beam cross over,
the potential for reconfiguration and for scanning of the beams by controlling only the phase of the signals passing through the amplification system,
complete flexibility in terms of the allocation of radio frequency power to the beams, up to the limiting case of allocating all the power to a single beam, PA1 distributed power amplification using identical amplifiers, each beam receiving equal power from all amplifiers, to ensure efficiency in correct operation and tolerable degradation in the event of equipment failures, PA1 efficient use of the reflector to ensure minimal angular spacing between the beams or the areas in which the same frequencies are to be re-used with sufficient isolation,
low sidelobe potential in the coverage area to enable frequency re-use; low sidelobes outside the coverage area,
facility for dual polarization in some beams or in all beams.
Many solutions have previously been put forward to secure these various properties. In active array antennas, each radiating element contributes to all the beams and a low-level beam switching matrix enables an entire channel to be allocated to a beam. However, active array antennas have the following disadvantages: to achieve high gain it is necessary to use complex deployment; it is difficult to control the level of the sidelobes and of the grating lobes using identical amplifiers with fixed amplitudes; finally, the system is complex and of high mass.
Active array antennas magnified by one or more reflectors have also been proposed. In this case the feed array is disposed optimally between the reflector and its focus, which tends to spread the power of each beam over the major part of the feed array, with a small translation from one beam to another. The beams can be controlled by controlling the phase only and it is possible to use identical amplifiers excited uniformly as in an active array antenna. However, in systems of this kind featuring an offset relative to the focus, each beam uses only part of the reflector and the feed device, which leads to the use of larger reflectors than in focussed systems. What is more, controlling the sidelobes in coverage raises problems when phase control only is used and identical amplifiers are employed. Furthermore, spurious radiation occurs outside the coverage area, which leads to scanning loss and the possibility of interference problems. This system can provide only limited scanning, unlike normal arrays, and, finally, it is not possible to limit the use of dual polarization to one beam or to a few beams only without employing connections to all the individual feed elements.
The advantages of this second type of antenna reside in its simplicity and some of its disadvantages can be alleviated using reflector shaping.
Another proposal is to use the focussing reflector with a conventional multiport amplifier. A device of this kind is shown in FIGS. 1A and 1B.
An array 2 of 16 individual feed elements A, B, C, D, A', B', C', D', A", B", C", D", and A"', B"', C"', and D"' is disposed in the focal plane F of a paraboloid reflector 1. To generate a beam such as beam No. 1, marked F1, four elements A, B, C and D are used, excited uniformly. These elements may be horns, dipoles, microstrips or other types of radiating element. Beam No. 2, marked F2, adjacent beam No. 1 also uses four elements B, D, A' and C', the elements B and D being shared with beam No. 1; beam No. 3 uses the elements A', C', B', and D' and shares the elements A' and C' with beam No. 2, and so on. Thus four elements uniformly fed are used for each beam and the power allocated to a beam can be increased only by transferring more channels to these four beams using low-level switching at the corresponding beam input port.
The reflector is efficiently illuminated by four sources: the sidelobes are low and the close spacing of the beams which results from sharing feed elements results in a minimum drop in gain at the intersection of the four beam coverage zones.
The problem is to distribute and amplify the power from the beam inputs B1 through B9 to the 16 elements with minimum losses and using identical power amplifiers. It is also desirable for each beam to use all the amplifiers, each operating at the same optimal output power, independently of the power (number of channels) allocated to this beam at a given time.
A partial solution to this problem has been achieved recently through the use of a multiport amplifier fed by a low-loss beam forming matrix. As shown in FIG. 1B, the 16 elements are connected to a matrix of microwave circuits having 16 input ports and 16 output ports and which is so constituted that if its 16 input ports are fed with signals having equal amplitudes and subject to a particular phase law, all of the power will exit through a particular output port. Butler matrices are conventionally used for this purpose. These matrices are such that the particular phase laws mentioned above are linear or stepped with levels which are multiples of pi/16. A matrix of this kind comprises four layers of hybrid dividers and its distribution (or transfer) matrix is unitary (no losses) and orthogonal.
This 16.times.16 matrix is fed by 16 identical amplifiers connected to an identical 16.times.16 matrix operating at a low power level. In this configuration comprising two Butler or similar matrices back-to-back, one input port of the input matrix corresponds to one output port (feed element) of the output matrix. In this case the beams are obtained by means of a low-level divider which has 9 input ports and 16 output ports, each beam port being connected to four input ports of the low-level Butler matrix.
These arrangements enable the power for each beam to be divided between 16 identical amplifiers. However, this system has the following disadvantages:
Because there is for each beam not a single feed (A) but four (A, B, C, D), it is necessary to activate four input ports rather than a single input port to excite each beam (No. 1). For this reason the signals in each amplifier are constituted by superposing four signals from four different inputs.
These signals have the same amplitude but their relative phases are different in the different amplifiers with the result that for some beams there is an amplitude ripple between the 16 amplifiers, which therefore do not all operate at exactly the same level, with optimal efficiency. Note, however, that if multiple beams operate at different frequencis the overall ripple is reduced by the averaging effect.
Similarly, if a global beam must be generated covering all of the area, all of the input ports of the first Butler matrix are fed resulting in significant ripple at the amplifiers.
The two back-to-back matrices each employ four layers of eight hybrid couplers so that each amplified signal passes through four couplers, involving losses and sensitivity to coupler inaccuracies.
In a 2.sup.N .times.2.sup.N matrix, each signal passes through N couplers and the total number of couplers for both matrices will be N.times.2.sup.N, the resulting complexity limiting the system to eight or 16 feeds at most.
Amplifiers with multiple inputs and multiple outputs and hybrid circuits with multiple inputs and multiple outputs are described in detail in, for example, the article by EGAMI and KAWAI in IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATION, Vol. SAC-5 No. 4, May 1987, pages 630 through 636.
Other solutions have also been proposed; one proposal, for example, is to use lenses instead of the multiport amplifier, but these devices have higher losses and higher mass. However, such devices could be used to create a large number of beams at higher frequencies, as in this case matrix systems are too complex and too lossy.
U.S. Pat. No. 4,901,085 in the name Spring et al., filed Sept. 23, 1988 and published Feb. 13, 1990, describes a configuration for a multibeam antenna feed system comprising a plurality of small and preferably identical hybrid matrix power amplifiers (HMPA).
Each HMPA, which comprises an input matrix and an output matrix interconnected by power amplifiers, is disposed between a low-level beam forming network and the radiating elements.
This configuration comprises a set of amplifiers disposed between the input and output matrices characterized by mirrow symmetry. A structure of this kind, which implies duplication of the matrices, is therefore relatively complex, bulky and heavy (important characteristics in the case of a satellite antenna).
Secondly, in the configuration described in this patent, the beam forming network connects each beam selector port to certain HMPA input ports. The amplifiers are then not always loaded identically, reducing the efficiency of the system, as can be seen from Tables 1, 2 and 3 of the specification of this patent.
Finally, the system described by this prior art document does not allow pointing of the beam while maintaining a constant loading of the amplifiers, which is a highly desirable characteristic for satellite communication antennas.
The present invention concerns a multibeam antenna feed device with which all these disadvantages can be alleviated and all of the properties mentioned above can be achieved.